1. Field of the Invention
The present invention relates to a power supply unit for supplying electric power to a load circuit.
2. Description of the Prior Art
FIG. 5 shows a conventional constant-voltage power supply unit. This power supply unit is constructed and operates as follows. Between a power source line 36 having a voltage V.sub.CC and a ground potential point (reference potential point), a pnp-type output transistor Tr and a first and a second resistor R11 and R12 are connected in series in this order from the power source line 36. The node a between the collector of the output transistor Tr and the first resistor R11 is connected to an output terminal 34.
The node b between the first and second resistors R11 and R12 is connected to the non-inverting input terminal (+) of a comparator 33. The inverting input terminal (-) of the comparator 33 is connected to the node d between one end of a resistor R.sub.0 and one end of a starting switch 32. The other end of the resistor R.sub.0 is connected to the ground potential point, and the other end of the starting switch 32 is connected through a constant current source 31 to the power source line. Assume that, when the starting switch 32 is turned on, the voltage at the node d is V.sub.11. To the output terminal 34, a capacitor 35 is connected to prevent oscillation.
A load 37 is connected to the output terminal 34. In reality, the capacitance of the load 37 acts as another capacitor connected in parallel with the capacitor 35. Here, however, for simplicity's sake, it is assumed that the capacitor 35 includes the capacitance of the load 37. The entire power supply unit starts operating when the switch 32 is turned on.
When the switch 32 is turned on, the voltage V.sub.11 at the node d is applied to the inverting input terminal (-) of the comparator 33. This causes the comparator 33 to output a low level, turning on the output transistor Tr. As a result, the capacitor 35 is charged quickly. When the voltage at the node b exceeds V.sub.11, the comparator 33 outputs a high level, turning off the output transistor Tr. However, as more current is supplied to the load 37, the voltage at the node b becomes lower until eventually the output transistor Tr is turned on again. In this way, the output transistor Tr is so controlled as to keep the voltage at the node b, thus the voltage at the output terminal 34, constant.
In this conventional power supply unit, as soon as the switch 32 is turned on to start up the unit, a current as high as the unit permits flows through the capacitor 35. As a result, as the characteristic curve in FIG. 6 shows, the power supply unit at first outputs its maximum current Imax that is higher than the current I.sub.0 that should normally be supplied to the load, and, only after that, the output current, tracing the characteristic curve from point A to B, to C, and eventually to D, settles to its normal value.
Inconveniently, such a flow of maximum current Imax at the start-up makes the source of the output current, that is, the power source line 36, unstable. This may lead to malfunctioning of other circuits 38 and 39 that are connected to the same power source line 36. For example, if the circuit 38 or 39 contains a microcomputer, the microcomputer may be reset unexpectedly.